Satoshi Tamate

Centrifuge Model Tests on Seismic Slope Failure

In this study, experimental analyses were carried out to investigate characteristics of slope failure by earthquakes and potential risks of secondary failure. A 1/50 scale model slopes were made of Kanto loam to simulate the shallow unsaturated soil in slopes. Seismic acceleration was applied on a shaking table in centrifuge, so that shear strain in the shallow unsaturated soil increased as the number of seismic cycles. It was found that the slopes failed as a result of accumulation of the plastic shear strain. In addition, changes in height and angle of slopes were measured in the upper remaining section after seismic failures. The potential risks of secondary failure (R v ) were analyzed using Taylor’s stability chart. It was ensured that the value of R v on the upper section decreased as the collapsed soil increased.

A Full-Scale Model Test for Predicting Collapse Time Using Displacement of Slope Surface During Slope Cutting Work

It is important to predict an onset of slope failures or rock falls for the occupational safety because about 15–20 workers were killed by these every year in Japan. Approximately half of the victims suffered from slope failure during slope excavation construction. In this research, in order to predict the time of slope failure during an experimental testing on a full-scale model slope was conducted, and the displacements of the slope surface were monitored during the slope excavation. The surface displacements rapidly increased with the elapsed time after the excavation, and the relationship between the displacements and the elapsed time included an exponential function just before the collapsed. We validated that the time of slope failure could be predicted by the relationship between the acceleration and the velocity of the obtained slope surface displacements. However, in order to predict the time of collapse, the data was required to compute only 2 s before the collapse. Therefore, we realised the importance of providing advisory and warning signal to give workers enough time to escape the slope failures. We have discovered that by computing the inverse of velocity of slope surface displacement, advisory and warning signals can be provided 2 min before the collapse.

CENTRIFUGE MODEL TESTS ON INSTABILITY OF AUTOMOTIVE PILE DRIVERS

The automotive pile driver is a piece of large-scale construction machinery. This machinery is used for ground improvement and foundation work. In recent years, there have been some reports of accidents in which the machinery overturned. In this study, a 1/25 scale model of an automotive pile driver was made to clarify the mechanism making the pile driver unstable, and centrifuge tests were performed. The load of each axle and the behavior of the model were measured in a centrifuge test. The grounds used with the model are two types of urethane foam with different strengths, with conditions imitating the uniform ground and non-uniform ground. As a result, the response acceleration did not have a significant difference due to differences in ground conditions. On the other hand, the effect of the ground condition was different in the axle load, and it was possible to evaluable the instability of the automotive pile driver when self propelling.

A Case Study on the Overturning of Drill Rigs on Construction Sites

Drill rigs are large pieces of construction machinery used to build pile foundations and to improve the ground stability. Crawlers comprising the lower base enable the machines to propel themselves. A tall leader given in the upper structure provides top heavy weight distribution. Sufficient bearing capacity and flatness are required in the bearing ground to keep the machinery stable without any tilts while it is propelling itself. Nevertheless, overturning often occurs at constructions. This study focuses on the phenomenon of ground instability causing machines to overturn. A drill rig weighing 372.5kN and with a leader 15.9m tall overturned on a building construction site. This machinery moved within the site to the positions required for building the diaphragm walls. This paper, firstly, summarizes operations prior to the accident. Secondly, the equilibrium condition of the machinery was calculated to clarify the stability, and the pressure acting on the ground through the crawlers was estimated. Ground properties were also investigated to assess the potential risk of failure in bearing ground. Finally, problems with the stability of drill rigs are discussed.

INSTABILITY OF MOBILE CRANES DUE TO THE GROUND PENETRATION OF OUTRIGGERS CAUSED BY BRITTLE FAILURE OF THE GROUND

ホイールクレーンなどの移動式クレーンは, アウトリガによって作業中の機体を安定させる. しかし, これが地盤に沈下したことによる転倒災害が多く発生している. 本研究では支持地盤の破壊によるアウトリガの沈下挙動に着目し, その破壊沈下特性の違いが移動式クレーンの転倒に与える影響を遠心模型実験と数値計算によって検討した. その結果, 表層が固結した2層地盤ではその脆性的な破壊によって, アウトリガが急激に沈下するために移動式クレーンは動的に不安定化し, 静的転倒角に比べて少ない機体傾斜でも転倒することがわかった. そのため移動式クレーンの転倒危険度を, 地盤破壊危険度と運動学的沈下危険度の最大値によって評価することを提案し, 地盤工学的な使用限界を示した.